Electronics Questions Answer the following with the MOST CORRECT answer. 1. The cathode end terminal of a semiconductor diode can be identified by: a. the negative sign marked on the case b. a circular band marked on the case c. an ohm symbol marked on the case d. all of the above 2. When a good diode is forward biased: a. its forward voltage is high b. it blocks the flow of current c. its forward resistance is high d. it can conduct current 3. The arrow of a semiconductor diode: a. is the cathode end of the diode b. indicates the current can be varied c. points in the direction of electron flow d. points in the direction of conventional current flow 4. A good diode has: a. high forward current and low reverse current b. low forward current and high reverse current c. high forward current and high reverse current d. low forward resistance and high reverse resistance 5. The voltage drop across a good germanium diode when completely forward biased is: a. 0.3 volts b. 0.5 volts to 0.7 volts c. 5.0 volts to 7.0 volts d. 1 volt 6. A diode is forward biased when: a. the cathode is negative with respect to the anode b. the anode is positive with respect to the cathode c. both (a) and (b) d. neither (a) or (b) 7. A Zener diode differs from a conventional silicon diode: a. in the forward biased direction b. in the reverse biased direction c. in both the forward and reverse biased directions d. in neither the forward or the reverse biased directions
7 Electronics Questions Answer the following with the MOST CORRECT answer. 8. A Zener diode can be used: a. a voltage limiter b. a voltage regulator c. a voltage reference d. all of the above 9. The Zener voltage refers to: a. the forward breakdown voltage b. the reverse breakdown voltage c. either (a) or (b) d. neither (a) or (b) 10. The resistance of a Zener diode in the breakdown region is: a. very large b. very small c. medium d. zero 11. The purpose of a rectifier is to: a. convert AC to pulsating DC b. convert DC to pulsating AC c. convert pulsating DC to DC d. none of the above 12. In a half -wave rectifier: a. each alternation of the input cycle is passed b. the negative alternation of the input cycle is passed c. the positive alternation of the input cycle is blocked by the diode d. each positive alternation of the input cycle is passed 13. In a full wave rectifier: a. each alternation of the input cycle is passed b. each positive alternation of the input cycle is passed c. each negative alternation of the input cycle is passed d. all of the above 14. Which of the following sine wave conversion statements is true? a. 28 volts peak = 19.9 volts rms b. 14 volts rms = 39.8 volts peak to peak c. 8.9 volts rms = 6.3 volts rms d. 6.3 volts rms = 17.8 volts peak to peak
Electronics Questions Answer the following with the MOST CORRECT answer. 15. The output ripple frequency of a full -wave rectifier is always: a. the same as the input frequency b. twice the input frequency c. half the input frequency d. 180 Hz 16. One of the following statements is true about half and full-wave rectifiers for the same AC input voltage: a. the peak output voltage of a full-wave rectifier is double that of a half-wave rectifier b. the peak to peak voltage of a half-wave rectifier is half of a full-wave rectifier c. the average output voltage of a half-wave rectifier is double that of a full-wave rectifier d. the output ripple of a full-wave rectifier is easier to filter, because it has twice the frequency of a half-wave rectifier 17. What kind of conduction characteristics does an SCR have? a. bi-directional b. omni-directional c. uni-directional d. none of the above 18. The voltage drop across a conducting SCR will be approximately: a. very high b. 6 volts DC c. zero d. 0.7 volts 19. The minimum value of current required to prevent an SCR from dropping out of conducting is known as: a. anode current b. holding current c. pull-in current d. minimum current 20. In reference to a transistor, a common- emitter circuit: a. does not provide voltage amplification b. is called the emitter follower c. will have a low resistance input d. provides the highest power amplification
9 Electronics Week # 3 21. Which of the following is used to change DC to AC voltages? a. rectifier b. capacitor c. inverter d. reactor 22. How many diodes are required for a full wave split phase rectifier? a. one b. two c. four d. six 23. What of the following does a DC voltmeter read? a. RMS voltage b. peak voltage c. average voltage d. effective voltage 24. In a split phase rectifier, if one of the diodes were to become open circuited, the output voltage would: a. approach the supply AC voltage b. drop to zero volts c. be Y:z wave rectified d. be Y. wave rectified 25. A DC voltmeter connected across a 60 Hz 120 volt AC supply will read: a. 108 volts b. 54 volts c. 170 volts d. zero volts 26. The maximum reverse voltage that may be applied to a rectifier diode is referred to as: a. RMS b. PIV c. AVE d. SCR 27. A full wave rectifier has 60 Hz 120 volts applied to its input. The output will have a ripple frequency of: a. 60 cycles per second b. 120 cycles per second c. zero d. 180 Hz 28. A single-phase half-wave rectifier is to have 240 volts applied toits input. What should be the value of its PIV? a. 240 volts b. 480 volts c. 339 volts d. 108 volts
29. What is the main purpose of a power supply filter? a. to boost the AC supply b. to "smooth" out the DC ripple voltage c. to block unnecessary power surges d. to regulate the AC supply Electronics Week # 3 30. Which of the following is most commonly used as a power supply filter? a. inductor b. capacitor c. resistor d. vacuum tube 31. Capacitance is the property of an electrical circuit that opposes: a. change in resistance b. change in current c. change in power d. change in voltage 32. A 10 Il f capacitor in series with a 100 KQ resistor has a time constant of: a. 10 seconds b. 100 seconds c. 0.1 seconds d. 1 second 33. Inductance is the property of an electrical circuit that opposes: a. change in resistance b. change in current c. change in voltage d. change in power 34. Which lead of a bipolar transistor carries the largest amount of current? a. collector b. emitter c. base d. none of the above 35. Bipolar transistors may be classified as: a. NPN and PIN devices b. NPNand PNP devices c. NNP and PPN devices d. N-type and P-type devices 36. Bipolar transistors have: a. two PN junctions b. one PN junction c. three PN junctions d. no PN junctions
11 Electronics Week # 3 37. A transistor's emitter-base junction must always be: a. subjected to a high reverse current b. biased the same as the collector-base junction c. forward-biased d. reverse-biased 38. A transistor's collector-base junction must always be: a. subjected to a high forward current b. reverse-biased c. forward-biased d. subjected to a low forward current 39. A transistor's collector current is always: a. much lower than its emitter current b. lower than its base current c. slightly lower than its emitter current d. higher than its emitter current 40. When checking a good transistor with an ohmmeter, the device should exhibit a: a. small difference between forward and reverse resistance across each junction b. large difference between forward and reverse resistance across each junction c. high resistance in both directions across its emitter junction d. low resistance in both directions across its collector junction 41. When a transistor has a very low and almost equal forward and reverse resistance between its emitter and base leads, the device effectively has: a. a good emitter junction b. a shorted collector junction c. a shorted emitter junction d. an open emitter junction 42. When a transistor has an infinitely high forward and reverse resistance between its base and collector leads, the device effectively has: a. a good collector junction b. an open collector junction c. a shorted collector junction d. an open emitter junction 43. A basic transistor is defined as: a. an NPN 3 element device used to control current flow b. a two junction device used to control current flow c. a three terminal device d. a three element device used to control current flow 44. Referring to the schematic symbol for a transistor, the arrow is always located in the: a. base lead and points to the emitter lead b. emitter lead and points to N-type material c. emitter lead and points to the base lead d. collector lead and points to the N-type material
12 Electronics Week # 3 45. Where does a transistor dissipate most of its power in the form of heat? a. at the emitter junction b. at the collector junction c. evenly throughout the device d. within the emitter region 46. The junction FET has three leads which are referred to as the: a. emitter, base and collector b. emitter, gate and collector c. source, gate and collector d. source, gate and drain 47. The arrow in the symbol for the FET is always located in the: a. emitter lead b. source lead c. gate lead d. drain lead 48. An SCR will switch to the on state when: a. its gate current is zero b. its forward breakover voltage is exceeded c. its reverse breakdown voltage is exceeded d. its gate current is below a certain threshold value 49. An SCR will switch to its off state when its: a. gate current is reduced b. gate current is increased c. forward current is increased to a sufficiently high value d. forward current drop below a level which will sustain conduction 50. An SCR is normally biased so it can be turned on by: a. momentarily increasing its forward voltage b. increasing its reverse voltage c. momentarily applying a gate current d. momentarily decreasing its gate current 51. A TRIAC can control current flowing in either direction through its: a. MT2 and gate terminals b. MIl and MT2 terminals c. MIl and gate terminals d. cathode and anode terminals 52. The TRIAC can be turned on in either direction by a: a. positive gate voltage but not a negative gate voltage b. gate current that flows out of the gate terminal only c. gate current that flows into the gate terminal only d. sufficiently high gate current that flows in either direction
1 3 Electronics Week # 3 53. The TRIAC will switch to the off state when its main operating current ( in either direction) drops below a specified value known as: a. breakdown current b. threshold current c. holding current d. leakage current 54. A device that is commonly used to trigger or "fire" a TRIAC is known as: a. bipolar transistor b. triggering thyristor c. SCR d. DIAC 55. When its Vp value is reached, the UJT will turn on and conduct between which of its terminals? a. emitter and base 1 b. emitter and base 2 c. base 1 and base 2 d. anode and cathode 56. The device in the thyristor family that can control current equally in both directions is the: a. SCR b. TRIAC c. DIAC d. PUT 57. The thyristor device capable of handling the largest power surges is the: a. SCR b. TRIAC c. DIAC d. UJT 58. The thyristor device that is equivalent of2 paralleled SCR's is the: a. DIAC b. TRIAC c. UJT d. PUT 59. The thyristor device that is used as a switching device and not as an amplifier is the: a. TRIAC b. DIAC c. UJT d. all of the above 60. The UJT has three leads: a. base, emitter and collector b. gate, source and drain c. emitter, base 1 and base 2 d. base, emitter 1 and emitter 2
1 4 Electronics Week # 3 61. The thyristor that has its leads referred to as the gate, cathode, and the anode is the: a. SCRonly b. DIAC only c. TRIAC only d. both a and b 62. The resistance of a photoconductive cell will be extremely high when: a. it is exposed to a high intensity light source b. no light strikes its surface c. it is reverse biased d. it is forward biased 63. The photovoltaic cell responds to changes in light intensity by: a. varying its transconductance b. changing its conductivity c. changing its resistance d. generating an output voltage 64. Under normal operating conditions, the forward voltage across an LED would most likely be between: a. 7 and 10 volts b. 3 and 5 volts c. 1 and 2 volts d. zero and 1 volt 65. The efficiency of an LED is approximately: a. 10% b. 40% c. 60% d. 90% 66. An R-C time constant is the time it takes a capacitor to charge to what percentage of the supply voltage? a. 37% b. 99% c. 70.7% d. 63%
1 5 Week#3 Fire Extine:uisher Tvpes On your Exam you may be asked a question regarding fire extinguishers. For this reason, I have provided this list of the classes of fire extinguishers, the symbol for the extinguisher and the use of the extinguisher. Class Symbol IYne of Fire to be used on A Green triangle Ordinary combustibles B Red square Flammable Liquids C Blue circle Electrical Equipment D Yellow star Combustible Metals Electronics Diode The diode is the most common semi-conductor device used. It is a simple PN junction. The symbol for the diode has the arrow pointing in the direction of conventional current flow. The arrow represents the anode and the line represents the cathode. The diode blocks current when it is reverse bias and allows CUtTent flow when it is forward bias. There are TWO major materials used for the construction of diodes, silicon (0.7 volts drop) and germanium (0.3 volts drop). There are many uses for the diode but the most common use is for rectification. When testing a diode there will be high resistance in one direction and very low resistance in the other direction. Zener Diode The Zener diode is designed to operate in the breakdown region of the diode curve. These diodes are most commonly used for voltage regulation, voltage references, voltage limiters or stabilizers. When this diode is biased permanently in the breakdown region, it's voltage remains virtually constant regardless of wide changes in current. This diode is used where a very constant voltage is required. An example of use would be the power supply for a computer CPU. Transistor Transistors are 3 lead devices having a collector, emitter and base. Transistors are most commonly used for amplification circuits. The best description for a transistor is a current valve. The amount of base current determines how much current will flow from collector to emitter. There are three transistor configurations: common base, common collector, and common emitter. The three configurations allow for different gain characteristics depending on its purpose in a circuit. Transistors are also used a switching devices provided both junctions are connected reverse bias,.
1 6 Week # 3 Electronics Thvristor The thyristor is a multi-layered semiconductor device that can be controlled OFF or ON by a control current. Once a thyristor has "fued" ( been turned on by a control current), it will continue to operate until the voltage across the thyristor is reduced to zero (in every AC cycle). There are tluee common types of tyristors: silicon controlled rectifier ( SCR), triac, anddiac. Silicon Controlled Rectifier (SCR) SCR's are a three terminal device consisting of an anode, cathode and a gate. When used as a rectifier, the SCR is capable of varying the DC output voltage to the load by controlling the point in each cycle at which the gate voltage is applied to the SCR. In other words controlling the timing in each cycle when the SCR is triggered or "fired". The SCR can be fired at different points in the AC cycle from 3 to 177 degrees. Remember, the SCR will continue to conduct until the voltage across it falls to zero. HOLDING CURRENT: is the amount of current flowing through the anode-cathode circuit needed to keep the SCR turned on. Triac The Triac is equivalent to two SCR's connected back to back. The triac can conduct on both halves of the AC input. The triac can be fired from 3 to 177 degrees and again from 183 to 357 degrees. Remember, the triac will conduct until the voltage across it reaches zero. When triacs are tested with an ohmmeter, they will have high resistance in both directions. Diac The diac is a device that will not begin to conduct until the voltage across it reaches the breakover voltage. When the breakover voltage is reached, the resistance across the device goes from a very high resistance to a very low resistance allowing current to flow though the diac. The diac will continue to conduct until the voltage across it reaches zero. Uniiunction Transistor The UJT is not a true transistor. It is more like a diode with two cathode connections. It works like a voltage controlled switch and does not amplify. The UJT has three leads: Basel, Base 2 and the emitter. In the UJT operation, normally a small current will flow from Base I to Base 2. When the voltage applied to the emitter reaches a certain threshold (several volts), the UJT will switch ON and a high current will flow from Basel to the emitter. When the voltage applied to the emitter falls below the threshold voltage, no current will flow from Base I to the emitter. Field-Effect Transistors FEr's have become more important than bi-polar transistors. They are easy to manufacture and have less silicon. There are two major FET families: the junction FET and the metal-oxide FET (MOSFET) Junction FET The two types of J-FET's are N-channel and P channel. The channel is like a silicon resistor that conducts current moving from the source to the drain. A voltage at the gate increases the channel resistance and reduces the drain to source current. Therefore, the J-FET can be used as an amplifier or a switch. Metal-Oxide FET The MOSFET has become the most important transistor. Most microcompu~er and memory circuits are an array of thousands ofmosfet's on a small sliver of silicon. All MOSFET's are N-type or P-type. The gate of a MOSFET has no electrical contact with the source and drain. A glass-like layer of silicon-dioxide (an insulator) separates the gate's metal contact from the rest of the transistor. A positive gate voltage attracts electrons to the region below the gate. This creates a thin N-type channel in the P-type silicon between the source and drain. Current can then flow through the channel. The gate voltage determines the resistance of the channel. This semiconductor device is very easily damaged by static electri~ity.
Week#3 Resistor Colour Code For the Exam it is also important to know the resistor colour code. You may be asked the value of a resistor by diagram or you may be given a resistor value and asked the colour code for that particular resistor. The colour code is as follows: colour first colour band second colour band X third colour band Black 0 0 Brown 1 1 10 Red 2 2 100 Orange 3 3 1000 Yellow 4 4 10 000 Green 5 5 100000 Blue 6 6 1000000 Violet 7 7 10000000 Gray 8 8 100000000 White 9 9 --------------- Fourth Colour band indicates tolerance as follows: Gold = + or - 5% Silver = + or - 10% No fourth band = + or - 20% Example: A 47 Kn resistor with a 5% tolerance would have the colour code of yellow, violet, orange, and gold. A resistor with the colour code of Red, Green, Red, silver would represent a 2.5Kn resistor. Note: This is the standard resistor colour code. Other resistor colour codes exist, for example the military colour code has five bands. However, this is the colour code that you are required to know for your Exam.